No one in Oli Khan’s family had ever lived in Scotland, or anywhere near it. But when, aged 23, Khan first set eyes on Linlithgow, a modest West Lothian town near Falkirk, he saw a prize greater than home. He saw opportunity. This chilly Scottish town – whose name means “lake in the damp hollow” – was the perfect place, Khan decided, to set up a curry house: it had a decent sized population, around 9,000 people, but no Indian restaurant. With help from his brother-in-law, who was in the restaurant trade in Birmingham, he opened his curry house in 1995 and named it Kismet – destiny.

Khan’s father, who arrived in Britain from Bangladesh as a waiter in 1962, had taught him that there was good money to be made in selling curry to the British, if you could adapt it to their taste for predictable sauces on a sliding scale of heat (mild korma, medium Madras, fiery vindaloo). For thousands of Bangladeshi immigrants in the 60s and 70s, working in Britain as OCs (“onion cutters”) and DCs (“dish cleaners”) was a way out of an even more precarious existence back home.

From the experience of his father, who worked his way up from waiting tables to owning a curry house in Kent, Khan knew that opening a new curry business anywhere in the UK was a low-risk proposition, because many locals, especially those who drank, would soon become hooked on “going for an Indian”. On the first night Kismet opened its doors, there were so many customers that by 7.30 all the food had run out.

Even as a child, growing up in the Sylhet region of Bangladesh – the ancestral home of around 80% of British curry chefs – Khan had understood that curry houses were an unshakeable part of the British way of life. By the late 1990s, curry had come to represent a newly cosmopolitan Britain, culminating in the former foreign secretary Robin Cook’s famous 2001 speech citing chicken tikka masala as the national dish of Britain, an example of the way that the country “absorbs and adapts external influences”.

Though hardly acknowledged by restaurant critics, except for mocking asides about their red flock wallpaper, curry houses were one of the great successes of the postwar restaurant industry in Britain. In her 2005 book Curry: A Biography, the historian Lizzie Collingham argued that the Sylheti curry cooks converted “unadventurous British palates” to a new flavour spectrum. Goodbye, mince and potatoes; hello, chicken bhuna. “More than any other ethnic food,” Collingham wrote, “the British have made curry their own.” Across the country, in any town big enough, you could guarantee that there would be at least one Kismet or equivalent, set up by enterprising Bangladeshi or Pakistani families. In high streets dominated by chains, from Harvester to Pizza Express, these were rare independent traders, and they played a role in the regeneration of many local economies in the 1980s and 90s. (In the past decade, according to a trade group, Britain’s curry houses employed 100,000 people and generated annual sales of £4.2 billion.)

Now, however, the curry house’s once unassailable place in British life looks precarious. Thousands of Indian restaurants are critically short of both staff to cook the food and customers to eat it. Across the industry, two or three curry houses are closing down a week.

This is a crisis with many causes, the effects of which extend far beyond curry. Since the Brexit vote and the subsequent collapse of the pound, independent food outlets of all kinds have been hurt by rises in rents, rates and food prices. Meanwhile, in families that run curry houses younger generations have moved away from catering to more lucrative jobs in medicine or tech. So long as there was a ready supply of new onion choppers from Asia, the exodus of upwardly mobile offspring did not affect curry houses too much. The real blow came when a harsh new politics of immigration came in, which made it harder for skilled south Asian chefs to work in the country, just as the wider British public were changing the ways in which they consumed curry.

Ever since the Conservative-Liberal Coalition assumed power in 2010, with David Cameron elected on an impossible pledge to reduce net migration into Britain to the “tens of thousands”, the Bangladeshi Caterers’ Association (BCA), of which Oli Khan is senior vice-president, has warned of a curry crisis. The BCA says that if nothing is done to support the industry, as many as a third of Britain’s curry houses – around 4,000 in total – will close over the next couple of years. Speaking in December, Lizzie Collingham told me she no longer saw “a bright future for these cheap and cheerful places”, even though curry as a dish (or series of dishes) is as beloved in Britain as ever.

A quarter of a century after he put his faith in Linlithgow, much of what Oli Khan knew about Indian food in the UK is crumbling. He sold Kismet and moved south several years ago when his father got cancer. Now, Khan owns two curry houses in the Hertfordshire commuter town of Stevenage and finds himself operating in a much tougher climate. At Surma, a takeaway he now runs, Oli Khan says that he is losing money on the £10.95 set dinner for one, yet he hesitates to put prices up because “everyone’s demanding more”. But Khan’s main complaint – in common with thousands of other curry house owners – is that he is permanently short-staffed.

In fact, Khanwas one of many curry house owners who backed Brexit in 2016, in the hope that exiting the EU would bring more favourable terms for south Asian immigrants. The president of the BCA, Pasha Khandaker, explicitly urged the association’s 4,000 members to vote leave, while the then employment minister, Priti Patel, told British Asian voters that by voting leave, they could “save our curry houses and join the rest of the world”. (In the end, about one in three British Asian people voted to leave the EU.)

Curry is still the soul food of the UK, the bowl of warmth people turn to when sniffy, sloshed or merely peckish

It is now dawning that under Theresa May’s government, the prospect of more Bangladeshi chefs being granted work visas is more distant than ever. A law that came into effect in April 2016 states that a skilled chef from south Asia must earn £35,000 or more a year, after deductions for accommodation and meals, to come work in a British restaurant that also offers a takeaway service. (The figure is only slightly lower for restaurants without takeaways.) Lord Bilimoria, the entrepreneur behind Cobra beer, is part of an all-party parliamentary group lobbying to reverse the law, which he calls “ridiculous” and “discriminatory”. No ordinary curry chef earns £35,000 (the industry average is £22-25,000 but the cooks who marinate lamb chops and mix raita get less).

Curry is still the soul food of the UK, the bowl of warmth that people turn to when sniffy, sloshed or merely peckish. But curry house owners have watched as dishes once unique to their restaurants were sold in cheapened forms, from supermarket ready meals – an “Indian banquet for 2” for only £9! – to the £6 Thursday curry nights offered by Wetherspoon pubs, which by 2016 was the biggest seller of curry in the UK. Another source of competition were the curries people had started to make for themselves at home, evidenced by rising sales of fresh ginger, chillis and exotic vegetables, the market for which leapt 22% to reach £215m in 2005. “You probably cook Indian food,” Oli Khan said to me, rather sadly, as we sat in his half-empty restaurant Spice Róuge on Stevenage high street, eating the Sunday night buffet.

For a few decades from the 1970s to the 2000s, the curry house – like its high street companion, the pub – looked like a permanent feature of British life; maybe even an emblem of Britishness itself. Yet it is now clear that our passionate relationship with these restaurants was a product of particular circumstances. For the high street curry house to flourish in its classical form, British tastes needed to stay fixed and south Asian cooks needed to be free to work here. Neither of these conditions now holds.

The curry house taught a white population that was eager to shed its colonial past to relinquish an earlier generation’s suspicion of garlic and chilli. For a while, curry lovers could tell themselves that openness about spice was a form of cultural broad mindedness. But the curry house’s current predicament shows that a national attachment to Indian food did not necessarily extend to the people who made it.

Oli Khan was one of many curry house owners who backed Brexit in 2016, in the misguided hope that exiting the EU would bring more favourable terms for south Asian immigrantsPhotograph: Glyn Kirk/AFP/Getty Images

* * *

Before anyone told me different, I thought curry house food was the most exotic and delicious in the world. As a child growing up in early 1980s Oxford, a trip to Uddin’s Manzil Tandoori restaurant on Walton Street, established 1971, was one of our great family treats.

Everything about Uddin’s was comforting and yet excitingly unlike how we ate at home: the stacked rounds of poppadoms, the pressed table linen, the stainless steel serving platters and candle-lit warmers to keep the food hot. Our parents would marvel at the charm of the dinner-jacketed waiters and the artistry of the rice: how did they get the grains in the pilau to turn so elegantly red? My sister and I were frugally encouraged to share a vegetable biryani – the yellow rice decorated with its slices of hard boiled egg and tomato – accompanied by a slightly oily mixed vegetable curry. We snuck spoonfuls of our mother’s sweet-creamy korma, and our father’s lamb Madras or vindaloo, squealing with pain as the chilli hit our tongues. It felt like visiting another country, but one that was strangely familiar. The whole dining room had a particular spicy smell that I couldn’t quite place.

When I was little, I had no inkling that these flavours and smells existed anywhere except Uddin’s on Walton Street. I was astounded when I discovered that this same food was served at other restaurants – not just in Oxford, but in other cities as well. There was the same biryani with the same hard-boiled egg garnish and the same sticky mango chutney in the same stainless steel bowls. I discovered that some versions of aloo gobi were greasier than others and there might be more or less delicacy to the seasoning, but the dishes were always essentially identical. In 1983, the trade publication Curry Magazine marvelled: “You are as certain to get the standard menu in the standard restaurant as you are to get a postage stamp from a post office whether you are in the coves of Cornwall or the Highlands of Scotland.”

As a nation, we started to take this cuisine and its comforts for granted. For anyone who lived through the curry house boom, it seemed such a part of the furniture of British life that it wasn’t obvious that it had only sprung into existence because of very specific economic and social forces. The curry house belonged to a Britain that was becoming affluent enough to eat out regularly for the first time, but not one where anyone had ever cooked an Ottolenghi recipe. Back then, if you wanted spicy food – to make a change from the bland Bisto of everyday life – you got an Indian.

In 1960, there were just 300 curry restaurants in Britain. The grandest of these was the venerable Veeraswamy’s off Regent Street, opened in 1926 by Edward Palmer, a retired army officer and Indian spice importer who had been the official caterer for the 1924 British Empire Exhibition at Wembley. Palmer aimed his menu at visiting Indian princes and upper-middle-class Londoners who appreciated the turbaned waiters and chandeliers.

Most “Indian” restaurants, however, started as fish and chip shops that sold a bit of curry and rice on the side, run by Bangladeshi boatmen from the predominantly Muslim district of Sylhet, which at that time was still part of British India.

This was not a region ever famed for its cuisine, which is one of the reasons that some British Asian people used to dismiss curry house food as the clumsy cooking of untrained sailors. However, food journalist Vikram Doctor, who writes for the Economic Times in Mumbai, told me that there were good cooks in Sylhet, namely the Mug or Mogh cooks who catered for the British Raj in Calcutta, whose great talent was to make inauthentic concoctions such as roast dinners jazzed up with masala spices. “I’ve always wondered,” Doctor told me last year in Mumbai, “whether the Mug cooks were among the first Sylhetis who cooked in British curry houses. It would make sense, if so, because they knew how to adapt a curry to please the British.”

Was any food ever so thoughtfully designed to please its audience as the early curry house menus? Each day, the chefs cooked up a gigantic vat of “base sauce” which could be adjusted to varying degrees of hotness and creaminess to suit the diners’ tastes. This sauce consisted of onions and carrots simmered with ginger, garlic, turmeric and other spices into an all-purpose orangey gravy. A dollop of this base sauce was sizzled in a pan of ghee, cooked meat and vegetables. If red peppers and green chillis were added, it became a jalfrezi. With yoghurt, garam masala and almonds, the same sauce transformed into a pasanda.

When the first significant wave of south Asian people settled in British towns in the 1960s, sometimes their neighbours complained that their cooking “stank”. Some white residents in Birmingham even went so far as to demand rate reductions from the city council to compensate them for the smell of spice. But slowly almost everyone was won over by the flavours. For those who really couldn’t stomach spice, there was usually an “English” section of the menu, offering an omelette and chips.

In the 1970s and 80s, more chefs arrived from Bangladesh. This was the golden era of the curry house, when its only real rival was long life Vesta curry, that peculiar amalgam of dehydrated mince in brown sauce. By 1980 there were roughly 3,000 curry houses. During the following decade, Manchester began to promote the largely Pakistani restaurants of its famous Curry Mile as part of a trade renaissance. One of the reasons to visit Birmingham was now the balti curry, invented by immigrants from Pakistan – a one-pot dish made with cumin, cloves and cassia bark, served with extra-large naan breads. Britain’s appetite for curry now looked as enduring as our love of tea and toast. By 1990, five years before Oli Khan set foot in Linlithgow, there were 6,600 curry houses (by 2011, there were 12,000).

An Indian restaurant in London in 1955.Photograph: Thurston Hopkins/Getty Images

This expansion was enabled by a wave of Bangladeshi immigration, especially after the country’s war of independence of 1971. In 1972, a new immigration act came into force decreeing that family members could only join husbands and fathers once the men were already settled in the UK. This meant that men came to Britain alone, and only later sent for their wives and children, which partly explains why curry house food never bore much resemblance to authentic cookery from the subcontinent. Home cooking in India and Bangladesh was generally done by women.

When he first joined his father in Britain aged 17, Oli Khan noticed that mouth-scorching British curries were nothing like the light, herb-rich food his mother made in Sylhet, with spices ground fresh on a stone for each meal. Traditional Bangladeshi cooking is a world away from creations such as the locally notorious “Widower” curry served at Bindi restaurant in Grantham, Lincolnshire, made from 20 Naga Infinity chillis, which is so hot that a customer who ate a portion in 2013 was left crying and hallucinating from the pain.

This blastingly hot food was an invented cuisine that no Indian person would fully recognise as their own. Given the choice, even the curry house owners themselves often preferred to go home and eat fresher, more subtle food than the dishes they served in their own restaurants. What they didn’t anticipate was that one day, some of their customers might start to feel the same way.

* * *

You can judge the age of a British curry restaurant from its name. If you see one that is called Taj Mahal, Passage to India or Koh-i-Noor (after the famous Indian diamond), it probably dates back to the first wave of curry houses in the 1960s.These eateries appealed to retired Old India Hands, who wanted to eat hot chutney and be treated like “sahibs” again. The names of 1970s curry houses began to shrug off the colonial past and evoke, instead, a vague sense of eastern exoticism: Lily Tandoori, Aladdin, Sheba – glamorous names to counteract longstanding British prejudices that south Asian food was malodorous and unclean. By the 1980s, however, such orientalism had also begun to seem hackneyed, and new restaurants opening in that decade often named themselves after ingredients, a more subtle form of rebranding: Tamarind, Cumin, or Lasan (Hindi for garlic).

Complaints that the British curry houses were inauthentic first began to be heard in the 1970s. In 1976, the cookery writer Madhur Jaffrey complained in her book An Invitation to Indian Cooking that these were “second-class establishments” serving a “generalised Indian food from no area whatsoever”. Jaffrey’s own books introduced millions of Britons to a more regional Indian cookery of flatbreads and complex dals and fresh, green coriander chutneys: a cuisine quite unlike the dear old tikkas in neon-red sauces. Increasing numbers of travellers came back from India with stories of meals of intensely varied flavours and textures that did not begin with a poppadom starter and a pint of Kingfisher. Curry houses went on the defensive, insisting that their menus were regional and “authentic”.

The objection that curry house food was inauthentic was true, but also unfair. It’s worth asking what “authenticity” really means in this context, given that people in India – like humans everywhere – do not themselves eat a perfectly “authentic” diet. When I asked dozens of people, while on a recent visit to India, about their favourite comfort food, most of them – whether from Delhi, Bangalore or Mumbai – told me that what they really loved to eat, especially when drinking beer, was something called Indian-Chinese food. It is nothing a Chinese person would recognise, consisting of gloopy dishes of meat and noodles, thick with cornflour and soy sauce, but spiced with green chillis and vinegar to please the national palate. Indian-Chinese food – just like British curry house food – offers a salty night away from the usual home cooking. The difference is that Indian people accept Indian-Chinese food for the ersatz joy that it is, whereas many British curry house customers seem to have believed that recipe for their Bombay potatoes really did come from Bombay, and felt affronted to discover that it did not.

The story of curry in modern Britain raises a question about food and identity. Does a cuisine belong to the people who eat it or the ones who cook it? The accusation of inauthenticity went hand in hand with a broader issue of ownership: once people realised that what they were eating was not proper Indian food, it was a short step to feeling that perhaps they, and not the south Asian chefs who cooked and served it, were the ones who owned it. Curry houses were always places where some customers behaved with a shocking sense of entitlement, occasionally expressed as outright racism. (One industry insider also told me that a surprising number of diners feel that no tip is required at a curry house.)

While the rest of Britain was assuming ownership of curry house food, many British Asian people were busy disowning it. As a student in Leeds in the 1990s, Nikita Gulhane, whose family arrived in the UK from India in the 1930s, occasionally ate at curry houses but found them to be “horrific places” full of drunken customers eating “shockingly overspiced food”. Gulhane is now a cookery teacher in London, whose courses include Keralan cuisine. He loves teaching people how to make simple dishes, such as roasting an aubergine over a flame and mashing it into a silky pulp with slivers of garlic and ginger – something quite alien to the curry house menu.

To many British Asian people, whether they had grown up in the UK or arrived as adults, British-Indian food seemed odd, both in form and flavour. The young cookery writer Meera Sodha, author of the 2016 cookbook Fresh India, always felt that curry house food was “misrepresentative” of the Gujarati cuisine she grew up eating at her mother’s table in Lincolnshire. The entire flavour profile of curry house curries, Gulhane told me, is founded on the tang of dried fenugreek leaf, a herb his mother never used, which acts as a flavour enhancer, comparable to MSG. (While talking to Gulhane, I suddenly realised that the spicy aroma at Uddin’s that I had found so distinctive and alluring as a child must have been fenugreek leaf.)

The chef Vivek Singh arrived in Britain in 2000 from Jaipur, where he had been cooking at the five-star Oberoi Rajvilas hotel. When Singh, who grew up in West Bengal, ate in a curry house in Canary Wharf, he found that he didn’t recognise any of the sauces. Singh was puzzled by the grid formula of the menu – pick any protein in any sauce – and the curious fantasy that a small kitchen serving just 60 or 70 covers could pretend to offer a hundred or more dishes. In a hugely successful departure from the traditional low-price British curry, Singh has been at the forefront of a new, high-end version of Indian food in Britain. At Singh’s Cinnamon Club in Westminster, which is a favourite of senior politicians, a main course of chargrilled halibut with bay leaves might cost £25.

When Singh first moved to Britain, he said, there were only five Indian restaurants here that he considered to be any good. Now, Singh claims, there are as many as 200 excellent examples, in which number he includes not just pricey establishments such as his own, but the more affordable London chain of Dishoom, modelled on the Irani cafes of Mumbai.

In Vivek Singh’s view, the original curry houses were based on a faulty understanding both of cuisine and of business. “You employ very few people. You don’t pay them very well. You are not willing to share your knowledge or techniques.” Isn’t it sad, I ask, that these old family-run businesses are now struggling? “Not for me!” replied Singh. “The British curry house is an institution that has to be revered. But it failed to adapt.”

Vivek Singh at his restaurant, The Cinnamon Club, in WestminsterPhotograph: PR Company Handout

* * *

Politicians have not exactly ignored the cries of crisis from curry house owners, but nor have they exactly helped. In 2012, David Cameron’s government unveiled a highly touted scheme to train unemployed British youth to cook the chicken tikka masalas that the nation loves so much. Five “curry colleges” were to be set up in Birmingham, Manchester, London and Leeds, funded by £1.75m in public money, to mitigate the harsh effects of the coalition’s immigration policy on curry kitchens.

The idea was to offer six weeks of training followed by a two-year apprenticeship on the job in which British chefs with no previous background in Indian food would learn how to cook everything from lamb kebabs to chapatis. One of the curry college lecturers, Dipna Anand, admitted to the BBC that it would be a challenge to teach people with no knowledge of south Asian cuisine the techniques and spicing that the south Asian cooks who set up Britain’s curry houses understood automatically. The real challenge turned out to be persuading the unemployed British people to apply. Out of 70 possible places, the curry colleges attracted just 25 students, of whom nine dropped out. The whole scheme folded within a year of being set up.

In hindsight, the curry college plan looks like an unusual fiasco, but the thinking behind it was in keeping with a wider mindset that saw curry as something so utterly British, it no longer required “foreign” input. Once it had been decided that curry was the quintessence of Britishness – and not even authentically Indian – it became very easy to argue that south Asian people were not needed to make it.

How difficult could it be to tandoori a chicken, when thousands of British people “made” their own curries every day from cook-in sauces? In 2015, George Osborne, standing in for Cameron during PMQs, replied to a question about curry houses closing under the pressure of immigration restrictions by saying: “We all enjoy a great British curry but we want the curry chefs to be trained in Britain.” It called to mind an old bit from the early 80s comedy sketch show Not the Nine O’Clock News, in which Rowan Atkinson plays a Tory politician speaking at his party conference: “I like curry, I do [pause]. But now that we’ve got the recipe, is there really any need for them to stay?”

Five years on from the curry college debacle, the BCA claims that there are now around 15,000 unfilled vacancies for curry chefs. Over the past few years, some of these jobs have been done by EU workers, from Italy and Eastern Europe, but after Brexit, there is no guarantee that this source of labour will continue. Meanwhile, most third-generation British Asian people have chosen not to follow their parents and grandparents into the curry business – and who could blame them, after witnessing their parents endure dwindling profits, long and anti-social hours, rude customers and racial abuse.

Opinion in the industry differs on whether you have to be south Asian to cook Indian food. At the Cinnamon Club, Vivek Singh trains British chefs of all races to make refined Indian food in his own apprenticeship scheme and calls it a “lazy myth” that “you have to be born of an Indian mother’s womb” to make Indian food. Then again, Singh’s expensive restaurants have a level of resources and manpower far beyond those of a small curry house. In theory, there is still no reason why British people should not learn to be Indian chefs, but it would take intensive training, and the aspiration to embark upon it, which few seem to have, given the low status accorded to the business.

British Indian cooking has always been far more skilled than its detractors appreciated. At the heart of it sits the tandoor, a cylindrical oven that is the engine of every curry restaurant. One winter lunchtime, I stood in the kitchen of Prana restaurant in Cambridge, with its 35-year-old owner, Kobir Ahmed. When I asked Ahmed why he still needed south Asian chefs in his kitchen, he pointed to the tandoor. Manning this vertical oven requires an adeptness at managing intense heat not seen in most European kitchens since the days of spit-roasting. It can take five years to learn how to use a tandoor, and even then, not everyone has the knack.

An experienced tandoor chef can slap six naan breads at a time in the depths of the oven without burning his hands, and juggle skewers of lamb and marinated cubes of paneer. An inexperienced chef would not be let anywhere near the oven, for fear of damaging either himself or the food. Ahmed pointed to one of his cooks, who was chopping vegetables nearby. “He’s been in the business 12 or 13 years and is still not ready to use the tandoor,” he said. The cook smiled and nodded.

Ahmed is a rare example of a third-generation British Bangladeshi restaurant owner, having left a promising first career as a community bank manager to set up Prana in 2016. (In what may represent yet another new phase in the naming of British Indian restaurants, the name, which means “cosmic energy” in Sanskrit, is often used in yoga.) On most curry-related questions, Ahmed vigorously disagrees with the older British Bangladeshis who run the BCA, such as Oli Khan.

It will be tricky to find capable chefs to man tandoors unless more south Asian chefs are allowed into the country

So far as Ahmed is concerned, the old curry houses were about “getting low-quality food and making it palatable”, whereas he would rather buy better-quality chicken and charge a bit more for it to keep his margins up. He told me there were Cambridge curry houses that had not put up their prices in 20 years because they were scared of losing customers. He pointed to a wall where he had hung a framed photograph of Andy Murray, next to a glass case containing one of the tennis balls Murray used when he won Wimbledon in 2013. “We want people to see that we have a winning mentality in everything we do,” he told me.

However, even a self-consciously modern Indian restaurant such as Prana – with its spiritual references and charred king prawn starters – is hampered by the shortage of chefs to cook south Asian food. Ahmed struggled to find a new head chef after his old one stopped work following an operation. Instead of more curry colleges – which Ahmed considered bogus – he would like to see the government putting money into training people on the job, whether they are of south Asian or European backgrounds. But there is also no getting around the fact that it is going to be tricky to find capable chefs to man those tandoors, unless more south Asian chefs are allowed into the country to work.

Curry houses did such a successful job of converting the British to a different way of eating that we found it easy to forget that we owe those flavours to the people who first cooked them for us. “I sometimes think we are a very ungrateful nation,” said Lord Bilimoria. “You are damaging an industry that provided food your country loves.”

Argy Bhaji restaurant in Rusholme, in 2000. The area is the curry capital of Manchester.Photograph: Nigel Hillier/UNP/United Northern Photographers

* * *

Maybe one of the reasons that the plight of the curry house has not received more attention is that to the average punter looking for a meal out, “Indian” is now just one option among many. Around a fifth of all British restaurants now serve various Asian cuisines. The generation that grew up eating chicken tikka became crazy for Vietnamese phô and Korean kimchi, for Japanese katsu curry and Szechuan pork, among others. The curry house transformed the way the British eat, coaxing the nation to enjoy spice and heat, only for many of its people to realise that they could get that spice and heat in a hundred different ways. The wider British public also started to see that Indian food itself was not one cuisine, but many.

In Bristol, Jim Pizer – who started out on a stall at music festivals – serves regional Indian food at the five busy branches of his Thali Cafe chain. Unlike the traditional curry houses, Pizer has not found his margins to be squeezed or his takings down in recent years. Since establishing a relationship with the online delivery service Deliveroo in 2015, he can hardly keep up with demand. But both his food and his business model differ completely from the “old format” restaurants, as he calls them. He sees no point in offering standard dishes that can be found cheaper in the supermarket, preferring to offer more unusual dishes such as pig cheek vindaloo and uttapam rice pancakes. Instead of a menu of 200 items, Thali Cafe offers thalis of rice, dal, vegetables, pickles and a main dish, all arranged on a single stainless steel plate. “There’s very little waste,” Pizer said.

Indian food in Britain used to mean south Asian immigrants cooks trying to cater for white British tastes; now it often means white Britons trying to reproduce authentic Indian recipes. Thali Cafe’s menus will state whether a dish belongs to the roadside stands of Mumbai or the beaches of Goa. Pizer, who is white, grew up in Birmingham. When he went for tea with his British Asian friends, he relished the food their mothers made, and he wanted to recreate those flavours in his restaurant. In the 17 years he has run Thali Cafe, Pizer has always employed women in his kitchens. “A lot of men forget what home cooking is like,” he said.

Pizer has proved there are still good profits to be made from Indian food in Britain. But when it comes to hiring and retaining staff, restaurants producing a more authentic take on regional cuisines face exactly the same problems as the old school curry houses. Pizer’s biggest headache is finding experienced Asian chefs. “There is a huge shortage of people at the moment,” he told me. Pizer is happy to employ people of other nationalities – one of his best chefs is Spanish – but he feels the food begins to lack authenticity when it is prepared without enough people whose roots are in south Asian cuisine. “It’s about knowing the way that chillis can vary in heat, or realising that you have to adjust the consistency of dal depending on the batch of lentils so that it isn’t too thick or too watery.”

Meera Sodha, who advises Pizer on recipes, agrees that “Indian cooking is a very particular, nuanced way of cooking” that is codified in the senses as much as in recipes. “If you haven’t grown up with a cuisine, it isn’t under the skin in quite the same way.”

All those years that we ate and enjoyed our Anglicised curry house curries, we had a glimmer that something far more wonderful and huge lay behind them. It was the food of India – the food of more than a billion people. Nikita Gulhane, the cookery teacher, told me that he wished Indian food could arrive in Britain all over again, on more equal terms, without the baggage of colonialism and migration and “going for an Indian”. Think about how excited people in London are about Korean food, he observed. If only we could taste Indian food for the first time, Gulhane continued, “People would think, ‘this is mental’.”

Here is one of the great ironies of the British curry house. After all those years of eating ersatz Indian food, the British palate matured to the point where diners would properly appreciate the artistry of the real thing, only to find that the real Indian chefs who could make it were denied the right to work here.

Not for the first time, British politics and British appetites are at loggerheads. In a speech in November, Theresa May claimed that curry was as dear to the nation’s heart as fish and chips, but her policies suggest otherwise. May’s government wants the UK to close its doors to new Indian chefs at just the moment we became capable of appreciating their work.

For 40 years or more, the British relationship with curry houses was part of our national self-image. It was an institution that flattered us that we were more tolerant and cosmopolitan than our meat-and-potatoes grandparents. If the sad decline of the curry house tells us anything, however, it is that open palates do not necessarily lead to open minds or open borders. Even Donald Trump advertises his love of tacos.

• Follow the Long Read on Twitter at @gdnlongread, or sign up to the long read weekly email here.